V-blenders are widely used in many industries requiring blending, granulating, and drying of powders. V-blenders (also referred to as twin shell blenders) are a type of tumbling mixer consisting of two hollow cylindrical shells or legs, usually of equal length joined at a 90 degree angle (FIG. 1). The mixing vessel is typically connected to a rotating shaft which causes a tumbling motion of the powders within the vessel or shell. The rotating shaft is usually parallel to the ground and perpendicular to the plane of symmetry of the blender. The V-blender may be fitted with an intensifier bar which rotates as much as 100 times the speed of the shell. The intensifier bar is typically positioned along the axis of rotation of the shell. V-blenders are used both in the laboratory as small-scale product development units and in manufacturing as large-scale production units.
Many existing V-blenders use constant speed tumbling motion to mix powders, e.g., V-blenders manufactured by Paul O. Abbe Inc. (Little Falls, N.J.), Bowers Process Equipment Inc. (Stafford, ON), Gemco (Middlesex, N.J.), Jaygo, Inc. (Mahwah, N.J.), Lowe Industries Inc., (Cadiz, Ky.), Patterson Industries Ltd. (Scarborough, ON), and Patterson-Kelley (East Stroudsburg, Pa.). In most cases, a mixture is considered well mixed when the standard deviation of samples taken from the mixture are equal to the standard deviation of a random mixture or fall within an acceptable variation for a particular application.
There have been numerous reports of incomplete or slow mixing in these devices. Gray found that a mixture of sand and ilmenite continued to improve its mixedness even after 1000 revolutions at 24 rpm. (Gray, J., "Solids Mixing Equipment", Chem. Eng. Progr., 53, (1957), 25). Wiedenbaum et al. found that a random mixture of same-sized sand and salt particles was not obtained even after 5000 revolutions at 24 rpm. (Weidenbaum, S. S., et al., "Mixing of Solids in a Twin Shell Blender", Ceramic Age, 79, (1963), 39) Chowhan and Linn found that it took approximately 1100 revolutions at 24 rpm to obtain a well mixed system of a cohesive drug with a free flowing excipient. (Chowhan, Z. T., et al., "Mixing of Pharmaceutical Solids, Powder Technology 24, (1979), 237) Cahn, et al. needed 1000 revolutions at 24 rpm to obtain a well mixed system of same sized CaCO.sub.3 and SiO.sub.2 particles. (Cahn, D. S., et al., "Blender Geometry in the Mixing of Solids", Ind. Eng. Chem., PD&D, 4, (1965), 318) Carstensen and Patel found that a system of same-sized lactose and cornstarch was not sufficiently mixed in 500 revolutions at 24 rpm. (Cartensen, J. T., et al., "Blending of Irregularly Shaped Particles, Powder Technology, 17, (1977), 273) Harnby found that a mixture of millet and salt exhibited significant segregation after 1000 revolutions at 33 rpm. (Harnby, N., "A Comparison of the Performance of Industrial Solids Mixers Using Segregating Materials, Powder Technology, 1, (1967) 94) Samyn and Murthy found that 118.mu. aspirin and 87.mu. lactose took 60 minutes to mix, although the rotation rate was not specified. (Samyn, J. C., et al., "Experiments in Powder Blending and Unblending, J. Pharm. Sci. 63, (1974) 371).
The main type of segregation in these experiments was found to be axial segregation. A conventional V-blender has no mechanism to induce flow in this direction, hence segregated regions may persist for long times. Three variations from the conventional V-blender may improve mixing by perturbing the axial flow. These include V-blenders with (1) legs of different lengths (P-K Cross-Flow.TM. Blender, Patterson Kelley, East Strousburg, Pa.), (2) the rotating shaft mounted parallel to the ground but offset from the orthogonal to the plane of symmetry of the blender (Challenger.TM. Offset.TM. V-Blender, Lowe Industries, Inc., Cadiz, Ky.), and (3) rotating blades mounted to rotate in the plane of the `V` of the blender (Chopper blades, Lowe Industries, Inc., Cadiz, Ky.)
An improved mixing method is disclosed in this patent application, consisting of a V-blender wherein mixing is enhanced by a controlled axial flow perturbation. As an example, perturbations are introduced by rocking the device with respect to its axis. Such perturbations produce a convective axial flow, resulting in large accelerations of the mixing process. It is claimed that similar enhancements could also be obtained by using other means to perturb the flow of particles.